Quantum interference and exceptional points.

Exceptional points (EPs), that is, branch point singularities of non-Hermitian Hamiltonians, are ubiquitous in optics. So far, the signatures of EPs have been mostly studied assuming classical light. In the passive parity-time (PT) optical coupler, a fingerprint of EPs resulting from the coalescence of two resonance modes is a qualitative change of the photon decay law, from damped Rabi-like oscillations to transparency, as the EP is crossed by increasing the loss rate. However, when probed by nonclassical states of light, quantum interference can hide EPs. Here it is shown that, under excitation with polarization-entangled two-photon states, the EP phase transition is smoothed until it disappears as the effective particle statistics are changed from bosonic to fermionic.